Use of control loops in communication devices are well known in the art. However, in a time division multiple access (TDMA) communication system where such a communication device may be employed, new problems with conventional control loops arise. For example, when the communication device is a transmitter in the TDMA system, the RF output of the transmitter must be gated "on" and "off." In addition, the power must be "ramped" up and down following a precise envelope shape in order to minimize the spurious output due to the gating of the RF power. The total time, including the ramp-down, off-time, and ramp-up, may only be on the order of tens of microseconds while the amplitude difference between the maximum and minimum output power may be greater than 40 dB. When a closed-loop RF power control system is implemented to accurately track an ideal reference waveform, the required-control loop must have both a broad bandwidth and large dynamic range. Practical considerations in the design of a transmitter with such a control loop recognizes that temperature and part-to-part tolerances degrade the control loop's ability to track the reference signal. Gain variation at the numerous RF stages within the transmitter also contribute to the degradation of the control loop's ability to track the reference signal.
Another problem with the simple first order control loop is that the bandwidth is easily affected by the gain in the many RF and baseband amplifier stages included in the control loop. In order to support the high bandwidth of the reference signal, the closed-loop bandwidth must be carefully controlled. As the amplifier gain or the RF input drive level changes, the closed-loop bandwidth changes, which in turn may cause the system to distort the RF envelope or become unstable.
The power control loop also may have problems handling the different gains that occur when each new transmitter is assembled with different parts. When the gain in the loop increases, the closed-loop bandwidth increases as well. This problem occurs where bandwidths of the individual elements of the loop (RF gain control device and detector bandwidth) will limit the maximum closed-loop bandwidth that the loop can support without becoming unstable. Design constraints of the detector (dynamic range versus detector open-loop bandwidth) tend to set the open-loop bandwidth of the system.
Thus a need exists for a control loop for use in a communication device which provides adequate loop performance in spite of, inter alia, temperature, part-to-part tolerances and gain variations at different stages of the communication device.